Background: Limited evidence is available about poly-victimisation (exposure to multiple forms of victimisation) and mental health among adolescents in low and lower-middle-income countries. The aim of this study was to examine the associations between lifetime exposure to poly-victimisation, health risk behaviours, symptoms of common mental health problems and suicidal ideas in the previous year among high school students in Vietnam. Methods: Participants were high school students in rural and urban districts of Hanoi, Vietnam. The data source was an anonymously-completed structured self-report survey. Lifetime exposure to poly-victimisation was assessed using the juvenile victimisation questionnaire revised 2 (JVQ R-2); mental health symptoms by the depression, anxiety and stress scale-21 (DASS-21); involvement in health risk behaviours and previous year suicidal thoughts and plans by questions adapted from the 2013 youth risk behaviour survey. Data were collected between October, 2013 and January, 2014 and were analysed using generalised structural equation modelling. Results: In total 1616/1745 (92.6 %) eligible students provided complete data. Prior year suicidal thoughts were reported by 21.4 % (95 % CI 18.5-24.5 %) of the female respondents and 7.9 % (95 % CI 6.2-9.8 %) of the male respondents. Prior year suicidal plans were reported by 7.8 % (95 % CI 5.9-9.8 %) of the girls and 4.0 % (95 % CI 2.7-5.3 %) of the boys. Poly-victimisation was associated with increased likelihood of involvement in health risk behaviours and symptoms of common mental health problems among both sexes, which increased adolescents' risk of having suicidal ideas in the previous year. Compared to non-victims or victims of fewer forms, poly-victims were also more likely to report suicidal thoughts and plans among both girls and boys (p < 0.05). Conclusions: Overall, the results revealed that poly-victimisation was associated with increased involvement in health risk behaviours, poorer mental health and increased risks of suicidal ideas among Vietnamese adolescents. Suicidal ideas were prevalent among the students. Interventions to assist victims of violence and prevention of violence, especially poly-victimisation, among adolescents in Vietnam is therefore important.